FIG. 1 depicts a rotary spectrometer 100 having ten input ports 101, 103, and 105, only three of which are depicted. As can be seen from FIG. 1, a first source of electromagnetic radiation 102 is coupled to a first input port 101 via an optical waveguide, a second source of electromagnetic radiation 104 is coupled to a second input port 103, and so on. Of course, each source 102, 104, 106 need not be embodied as a lamp. The sources 102, 104, 106 may be embodied as an LED, a laser, or any other source of electromagnetic emission. Further, each of the input ports 101, 103, and 105 may actually be coupled to a single source. In principle, a rotary spectrometer may be coupled to as many sources as it has input ports and to as few as a single source (and any number in between). For the sake of illustration, various embodiments of rotary spectrometers are depicted as being used in a setting in which each input port is coupled to a different source. As just explained, this is for explanatory purposes only and is not an essential part of the invention. A rotating body (not depicted) houses ten optical bandpass filters 108, 110, and 112, only three of which are depicted. The rotating body may also house one or more neutral density filters 114 and 116, which are aligned with the optical bandpass filters 108 and 110. The neutral density filters 114 and 116 may be aligned either in front of, or in back of, their respective bandpass filters 108 and 110. The rotating body is actuated under the control of a motor (not depicted). Rotation of the body causes the optical bandpass filters 108, 110, and 112 to come into alignment with the input ports 101, 103, or 105. As depicted in FIG. 1, optical bandpass filter 108 is in alignment with a first input port 101, which carries electromagnetic radiation from a first source of electromagnetic radiation 102. Similarly, optical bandpass filter 110 is in alignment with a second input port 103, which carries electromagnetic radiation from a second source of electromagnetic radiation 104, and so on.
In operation, electromagnetic radiation is emitted from the first source 102 and is carried by an optical waveguide (not depicted) to the first input port 101. The electromagnetic radiation propagates from the input port 101 to the optical bandpass filter 108 in alignment therewith. The optical bandpass filter 108 is a device that allows electromagnetic radiation within a passband to pass through, while attenuating electromagnetic radiation falling outside of the passband. Other types of filters include cut filters that can pass radiation below or above a particular wavelength (i.e., a highpass or lowpass filter).
The source of electromagnetic radiation may be a lamp, which may be used, for example, in a production, lab, or pilot-scale line to cure a substance. The lamp may exhibit a characteristic wavelength-energy profile, meaning that due to the chemical composition of the lamp, a relatively great amount of energy is carried on certain wavelengths, while a relatively scarce amount of energy is carried on other wavelengths. A neutral density filter 114 and 116 may be placed in alignment with an optical bandpass filter 108 and 110 having a passband that includes wavelengths expected to carry a relatively great amount of energy.
After propagation through the optical bandpass filter 108, 110, and 112, the electromagnetic radiation propagates toward, and is incident upon, a photoelectric element (not depicted), which reacts to incident electromagnetic radiation by exhibiting an electrical voltage. The voltage exhibited across the photoelectric element is approximately proportional to the intensity of the electromagnetic incident upon it. The photoelectric element is coupled to a detection circuit 118, 120, and 122, which amplifies the signal and may optionally digitize the signal for delivery to a computer system (not depicted). The detection circuits 118, 120, and 122 amplify their respective input signals in accordance with a gain factor, which may be selected, for example, by adjustment of a potentiometer interposed in the feedback path of an operational amplifier. One complete rotation of the body housing the optical bandpass filters 108, 110, and 112 and neutral density filters 114 and 116 achieves the effect of taking one measurement of each source 102, 104, and 106 at the wavelength ranges determined by the passbands of each of the bandpass filters 108, 110, 112. The computer system may be used, for example, to display information related to the intensity of electromagnetic radiation within the bandpass ranges exhibited by the bandpass filters 108, 110, and 112.
The above-described system should be designed so that the detection circuits 118, 120, and 122 utilize gain factors that are as large as possible without providing occasion for the detection circuits 118, 120, and 122 to saturate. Adherence to such a principle ensures that the greatest resolution of measurement is yielded from the analog-to-digital converters interposed between the detection circuits 118, 120, and 122 and the computer (the analog-to-detection circuits may be embedded within the detection circuits 1 18, 120, and 122, as described above).
The process of choosing an appropriate gain factor for each detection circuit 118, 120, and 122 is tedious. For example, consider the process of selecting an appropriate gain factor for the first detection circuit 118. The first bandpass filter 118 is indirectly optoelectrically coupled to the first source 102, which emits electromagnetic radiation having an intensity, I, which may be measured in eV/(area)(sec). The electromagnetic radiation propagates through the neutral density filter 114 that is in alignment with the first input port 101, whereupon all wavelengths are attenuated approximately equally by a factor, KA, meaning that the intensity exhibited at the output of the neutral density filter is equal to I/KA. Thereafter, the electromagnetic radiation is filtered by the first bandpass filter 108, so that only wavelengths falling within the passband are permitted to pass. Thus, at the output of the first bandpass filter, the intensity of the electromagnetic radiation is equal to IA/KA, where IA represents the intensity of electromagnetic radiation within the passband of the first bandpass filter 108. Of course, this is an ideal value, because not all wavelengths are passed with equal ease. For instance, it is easier to pass longer wavelengths than shorter wavelengths, meaning that a bandpass filter designed to pass shorter wavelengths will attenuate relatively more electromagnetic radiation falling within its passband to some extent. For present purposes, this effect is ignored, but further complicates setting the gains. Finally, the electromagnetic radiation is incident upon a photoelectric element (not depicted), whereupon it is converted into a voltage and amplified by a gain factor, GA, meaning that the output voltage of the first detection circuit is equal to [GA][IA/KA]. Herein, the photoelectric element is described as converting incident electromagnetic radiation into a voltage. Of course, a photoelectric element may convert incident electromagnetic radiation into an electrical current, as well. Such photoelectric elements are included within the scope of the invention. For the sake of illustration only, photoelectric elements are described herein as converting incident electromagnetic radiation into a votlage, although conversion into a current is equally within the scope of the invention. In order to satisfy the above-stated principle that the gain factor should be selected so as to be as large as possible without providing occasion for the detection circuit 118 to saturate, the following condition should be satisfied:[GA][IA/KA]≦max output,   (1)where max output represents the maximum output voltage of the linear region of the detection circuit.
As illustrated by FIG. 2, the condition to be satisfied changes with rotation of the body housing the optical bandpass filters 108, 110, and 112. FIG. 2 depicts the spectrometer of FIG. 1, after the body has been rotated so as to bring the tenth bandpass filter 112 into alignment with the first input port 101. Once again, the first bandpass filter 118 is indirectly optoelectrically coupled to the first source 102, which emits electromagnetic radiation having an intensity, I. The electromagnetic radiation from the first source 102 propagates to the tenth bandpass filter 112, whereupon it is filtered, so that only wavelengths falling within the passband are permitted to pass. Thus, at the output of the tenth bandpass filter 112, the intensity of the electromagnetic radiation is equal to IJ, where Ij represents the intensity of electromagnetic radiation within the passband of the tenth bandpass filter 112. Finally, the electromagnetic radiation is incident upon a photoelectric element (not depicted), whereupon it is converted into a voltage and amplified by a gain factor, GA, meaning that the output voltage of the first detection circuit is equal to [GA][IJ]. In order to satisfy the above-stated principle that gain factor should be selected so as to be as large as possible without providing occasion for the detection circuit 118 to saturate, the following condition should be satisfied:[GA][IJ]≦max output.   (2)
As can be seen, condition 2 differs from condition 1, illustrating the point that the appropriate gain factor is a function the orientation of the body housing the optical bandpass filters. Since there are as many conditions to be satisfied as there are optical bandpass filters housed in the body (i.e., ten conditions in the case of the spectrometer depicted in FIGS. 1 and 2), the gain factor must be set in light of each of the conditions. Thus, for each detection circuit 118, 120, and 122, the gain factor is chosen by rotating the body to each of the positions and setting the gain factor to the greatest value that is consistent with each of the conditions.
In addition to being tedious, the above-stated procedure exhibits another shortcoming. Specifically, the gain factor arrived at may be particularly unsuitable for bandpass filters having a bandpass range that passes wavelengths upon which the source emits relatively little energy. For such wavelengths, the selected gain results in a condition whereby only a fraction of the quantization range of the analog-to-digital converter is used. This leads to low-resolution measurement and enhanced susceptibility to noise, qualities that are inimical to proper functioning of a spectrometer. Moreover, the inclusion of neutral density filters (which are included for the sake of rendering the intensities of the electromagnetic radiation incident upon the various photoelectric elements into roughly similar range) has a drawback. Neutral density filters inhibit radiation that could otherwise be used to improve the accuracy and precision of the instrument from reaching the photoelectric elements—a result inimical to the goal of accuracy and precision.
As is evident from the foregoing, there exists a need for a scheme by which a gain factor may be selected in a simple manner, and yet may be suitable for any position of the body housing the optical bandpass filters. There further exists a need for elimination of neutral density filters.